A head cover attached to a cylinder head of an internal combustion engine to cover a valve mechanism is sometimes formed with a breather chamber (breather passage) on an inner side thereof such that a blow-by gas flows through the breather chamber in order to have an oil mist contained in the blow-by gas be separated from the blow-by gas (gas-liquid separation) (see Japanese Patent Application Laid-Open Publication No. 2005-155475, for example).
For the purpose of achieving a compact arrangement of an intake system of the internal combustion engine, an intake collection chamber of an intake manifold or a surge tank may be disposed close to an upper side of the head cover. In such a case, in order to cope with the requirements for a design change or volume increase of the intake collection chamber, it is demanded to reduce the height of the head cover and hence minimize the height of the breather chamber (passage) defined on the inner side of the head cover.
It should be noted in this regard that if the volume of the breather chamber were insufficient, the performance of the breather chamber to remove oil mist would decrease, and therefore, it is necessary to ensure an adequate volume of the breather chamber while maintaining a small height of the breather chamber in order to cope with the design change or volume increase of the intake collection chamber as well as achieve favorable oil mist removing function. This can be achieved by increasing the size of the breather chamber of the head cover in a lateral direction (in a horizontal direction perpendicular to a crankshaft direction of a longitudinal engine).
However, the size increase of the breather chamber of the head cover in the lateral direction can lead to a ceiling wall of the head cover having a large flat surface, and this can reduce a panel rigidity of the ceiling wall, which in turn makes the ceiling wall easier to resonate with vibrations generated by the valve mechanism and the like on the engine main body side, and thus deteriorates the performance regarding the vibration and noise phenomena (NVH performance).